Macroplastics as artificial substrates for macroinvertebrates in four subtropical river systems in the Eastern Cape, South Africa

Abstract The ecological implications of macroplastic accumulation in rivers, particularly their role as artificial substrates for aquatic organisms, remain poorly understood, especially in the Afrotropical streams. We investigated how macroinvertebrates colonise macroplastic substrates relative to natural substrates in four minimally disturbed headwater stream sites in the Eastern Cape, South Africa. Experimental substrate groups, including NS (100% natural substrate), NP (1:1 mix of natural and plastic substrates), and PD (100% plastic substrate), were deployed for 180 days. Macroinvertebrates colonised macroplastics and natural substrates, and overall community structure did not differ significantly amongst the substrate groups. However, the natural substrate supported higher taxonomic richness (41) than the macroplastic substrate (33). Temporal variation strongly influenced macroinvertebrate community assemblage, with colonisation significantly increasing over time, across all substrate groups. Taxon-specific preferences were also evident: macroinvertebrate taxa such as Glossosomatidae, Psephenidae, and Veliidae showed positive associations with plastic substrates, whereas Libellulidae, Tipulidae, and Tabanidae preferred natural substrates. These findings suggest that macroplastics serve as suitable artificial substrates for aquatic macroinvertebrates, but their ecological importance varies amongst taxa. Natural substrates still supported a higher number of taxa, though not statistically significant, underscoring the importance of preserving naturally occurring substrates as key habitats for freshwater biodiversity.